Process for making tailored polyetheylene resins for sheets

Abstract

This invention relates to coupling of polyethylene resins, more specifically coupling of polyethylene resins for use in extruded profiles, especially extruded profiles for sheet extrusion and cut sheet thermoforming applications and geomembranes. The process involves conveying a HDPE resin through an extruder, wherein the extruder comprises a feed zone, a first melt zone downstream of the feed zone, a second melt zone downstream of the first melt zone, and a third melt zone downstream of the second melt zone. The resin is melted in the first zone, contacted with oxygen in the second melt zone, and contacted antioxidant in the third melt zone.

What is claimed is: 
     
       1. A process for making an extruded sheet comprising:
 a) conveying a HDPE resin through an extruder, wherein the extruder comprises a feed zone, a first melt zone downstream of the feed zone, a second melt zone downstream of the first melt zone, and a third melt zone downstream of the second melt zone; 
 b) contacting the resin with a gaseous medium comprising oxygen in the second melt zone, under conditions sufficient to promote at least some long chain branching thereby producing a tailored HDPE; and wherein the HDPE resin used in step a) was polymerized using a catalyst comprising chromium; 
 c) adding an antioxidant(s) to the tailored HDPE in the third melt zone; 
 d) passing the resin resulting from step (c) through a die to form a sheet having a thickness in the range of 0.25 mm to 25 mm; and 
 wherein the tailored HDPE has a low shear-rate viscosity, measured at 190° C. and 0.01 sec −1 , that is from 50 percent to 340 percent higher than the low shear-rate viscosity, measured at 190° C. and 0.01 sec −1 , of the non-tailored HDPE. 
 
     
     
       2. The process of  claim 1 , wherein the resin resulting from step (c) is characterized as having at least a 17 percent increase in melt strength over the unmodified HDPE resin as determined by Rheotens at 190° C. 
     
     
       3. The process of  claim 2 , wherein the oxygen in the second melt zone is present in an amount from 5 percent to 21 percent of the gaseous medium. 
     
     
       4. The process of  claim 2 , wherein the oxygen in the second melt zone is present in an amount from 10 percent to 21 percent of the gaseous medium. 
     
     
       5. The process of  claim 2 , wherein the oxygen in the second melt zone is present in an amount from 18 percent to 21 percent of the gaseous medium. 
     
     
       6. The process of  claim 1 , wherein the temperature in the second melt zone is in the range of from 150° C. to 280° C. 
     
     
       7. The process of  claim 1 , wherein the resin is first formed into pellets then re-melted in order to form the sheet. 
     
     
       8. The process of  claim 1 , wherein the antioxidant is a primary antioxidant, and wherein the process further comprises adding a secondary antioxidant to the tailored HDPE at a point downstream of the second melt zone. 
     
     
       9. The process of  claim 8 , wherein the secondary antioxidant is a phosphite antioxidant, and is added in an amount from 100 ppm to 1500 ppm. 
     
     
       10. The process of  claim 1 , wherein the sheet is characterized as having a sag after heating, via infrared absorption for a period of 150 seconds, of less than 2.0 inch (50.8 mm), as measured by a light curtain when the sheet has the dimensions of 24 inch (610 mm) by 36 inch (914 mm) by 0.120 inch (3.05 mm) thick. 
     
     
       11. The process of  claim 10 , wherein the sheet is further characterized as having a sag less than 1.8 inch (45.7 mm) as measured by a light curtain. 
     
     
       12. The process of  claim 11 , wherein the HDPE resin used in step a) has a melt index, as determined by I21, from 8 g/10 min to 14 g/10 min. 
     
     
       13. The process of  claim 10 , wherein the sheet is further characterized as having a sag less than 1.7 inch (43.2 mm) as measured by a light curtain. 
     
     
       14. The process of  claim 10 , wherein the HDPE resin used in step a) has a density in the range from 0.947 g/cc to 0.953 g/cc. 
     
     
       15. The process of  claim 1 , wherein the HDPE resin used in step a) has a density in the range from 0.940 g/cc to 0.955 g/cc. 
     
     
       16. The process of  claim 15 , wherein the HDPE resin used in step a) has an I21/I2 in the range of from 90 to 240. 
     
     
       17. The process of  claim 1 , wherein the HDPE resin used in step a) has a melt index, as determined by I21, from 5 g/10 min to 20 g/10 min. 
     
     
       18. The process of  claim 1 , wherein the HDPE resin used in step a) has an I21/I2 in the range of from 75 to 200. 
     
     
       19. The process of  claim 1 , wherein the HDPE resin used in step a) is a copolymer comprising ethylene and an alpha olefin having from 3 to 10 carbon atoms. 
     
     
       20. The process of  claim 19 , where the alpha olefin is 1-hexene. 
     
     
       21. The process of  claim 1 , wherein the antioxidant is a phenolic antioxidant, and is added in an amount from 100 ppm to 1500 ppm. 
     
     
       22. The process of  claim 1 , wherein the HDPE resin used in step (a) was polymerized from a Ti modified CrO catalyst in a gas phase reactor, where the catalyst activation temperature was from 400° C. to 1000° C. 
     
     
       23. The process of  claim 22 , wherein the catalyst activation temperature was from 400° C. to 600° C. 
     
     
       24. The process of  claim 22 , wherein the catalyst activation temperature was from 601° C. to 750° C. 
     
     
       25. The process of  claim 22 , wherein the catalyst activation temperature was from 751° C. to 1000° C. 
     
     
       26. The process of  claim 22 , further characterized in that the polymerization temperature was from 95° C. to 100° C. 
     
     
       27. The process of  claim 26 , wherein the polymerization temperature was less than, or equal to, 95° C. 
     
     
       28. The process of  claim 26 , wherein the polymerization temperature was less than, or equal to, 90° C. 
     
     
       29. The process of  claim 26 , wherein the polymerization temperature was less than, or equal to, 85° C. 
     
     
       30. The process of  claim 1 , wherein at least one polyethylene glycol is added to the tailored HDPE in step c).